156 ARTHUE WILT.EY. 



the formation of protostigmata by abstriction is probably 

 primitive. From this point of view it may be concUided that 

 subdivision of protostigmata has dropped out of the 

 ontogeny of Clavelina, while abstriction of protostig'- 

 mata no longer occurs in Botryllus.^ 



Rectificatiok. 



In my former studies on the Protochordata, published in 

 this Journal,^ the main object of which was to obtain an inde- 

 pendent basis of comparison between the Ascidians and 

 Amphioxus, I referred incidentally to certain points which 

 did not fall within the strict limits of my thesis, among these 

 being the pericardium and heart of Ciona. With regard to 

 the cavity of the heart, it seemed to me to arise by a splitting 

 apart of the two layers of what 1 described as a pericardial 

 septum. In the memoir quoted above, Professor Julin denies 

 the existence of this septum, and says that the heart arises in 

 Ciona, as in all Ascidians, by invagination of the wall of the 

 pericardium. In a form such as Clave! in a, where a longitu- 

 dinal invagination of the dorsal Avail of the pericardium 

 occurs, the lips of the involution finally come together and 

 coalesce to form a longitudinal cardiac raphe. Now in 

 Ciona there is a ventral raphe as well as a dorsal one, as 

 shown in my fig. 30, PI. 31, vol. 34, and in some of Julin's 

 figures. If, as M. Julin states, the heart of Ciona arises by 

 invagination and not by the splitting of a septum, then it is 

 necessary to give an explanation of the double cardiac raphe 

 which has been observed in this Ascidian.^ 



The question as to the exact origin of the heart in Ciona 

 is not, however, the special matter to which the heading of this 

 section of the present paper refers. 



1 Garslaiig states (p. 511) lliat in tlie Poly&t^elid Tlijlacium sylvani 

 " eiglit protostigmata arise on eacli side of the pharynx, and become sub- 

 divided, in regular order from before backwards, to form a corresponding 

 number of rows of secondary stigmata," 



2 Vols. 34 and 35, 1893. 

 ^ See note on p. 158. 



